Astronauts returning from missions in space may take months to start recovering from dangerous bone-thinning.

Living in conditions of near zero-gravity places less stress on bones, and in response, they weaken.

This thinning could mean that astronauts are vulnerable to bone fractures.

Researchers have discovered that the bone-loss, far from stopping when they come back to Earth, can continue for months.

The study, presented in this week's Lancet medical journal, could have serious implications for any future attempts to travel longer distances in space.

Researchers from St Etienne in France closely examined 15 cosmonauts who had spent between one month and six on board the Mir space station.

The cosmonauts started losing bone density in the tibia - a weight-bearing leg bone - as soon as they reached Mir.

After a month at near-weightlessness in space, an average of 1.7% of bone density was lost.

This continued as the space missions lengthened.

Bone thinning continues

However, bone thinning even continued when they came back to Earth, suggesting that once the process had been set in motion, it is hard to stop it happening.

Dr Michael Holick, an expert on weightlessness and bone density, said: "Before the space programme can send astronauts for long-duration space flights, such as a mission to Mars or colonisation of the Moon, there needs to be a resolution to the difficulty of microgravity-induced bone loss."

The closest UK osteoporosis specialists come to this phenomenon is the treatment of patients who are on long-term bed rest.

Dr Brian Kaufman, based at the Central Middlesex Hospital in London, said: "One treatment that may be used is biphosphanates, which coat the bone and stop it being broken down."

Astronauts carry out rigorous physical training while in space to try to minimise the bone loss suffered.